Hot-melt adhesives are thermoplastic materials which are solid at ambient temperature and which, by heating (approximately 180° C.), become more or less viscous liquids. These liquids are applied to a first substrate and then the substrate is covered with a second surface. On cooling, adhesion between the substrate and the second surface is obtained. The open time is the period during which the adhesive which has been applied to a substrate which is at ambient temperature remains sticky, that is to say the time interval during which it is possible to apply the second surface and, on cooling, to obtain adhesion between the substrate and the second surface. Once this time limit of the open time has passed, it is no longer possible to obtain satisfactory adhesion between the substrate and the second surface. These adhesives can also be positioned in the solid state between the two materials to be adhesively bonded, then the combination is heated, the adhesive melts and, on cooling, the two materials are adhesively bonded to one another. This technique is used in the textile industry; the adhesive is in the form of a film or powder which is positioned between two fabrics and then adhesive bonding is brought about using a hot iron. These adhesives are denoted by the abbreviation HMA (hot melt adhesives) and are sometimes also denoted by the term “thermofusible adhesives”.
The polyamides conventionally used as thermofusible adhesives in the textile field (fixation of wovens and nonwovens), electronic field (coating of copper wires for reels) or motor vehicle field are applied at temperatures greater than their melting points and they adhere well to various surfaces. The resistance to heat of thermofusible adhesives is therefore limited to their melting point and the product often becomes soft and adhesive already at 10° C. below its melting point. Today, a demand is observed in all fields of application for an enhancement in the performance of thermofusible adhesives, such as, for example, better adhesion to treated surfaces, better resistances to heat or better resistance of the adhesive bonding to cleaning with steam or with drycleaning solvents. This improvement in the performances must be made without changing the technology used to apply them. One means for improving the performance of an HMA is to crosslink it after adhesive bonding.
The prior art has already disclosed the crosslinking of polyamides and copolyamides. Patents DE 3725486, EP 940 461 and WO 2002/026887 disclose crosslinking by isocyanates. Patents WO 2002/086009 and U.S. Pat. No. 6,515,048 disclose crosslinking by epoxy compounds. Patent EP 326 444 discloses crosslinking by a pre-encapsulated agent. U.S. Pat. No. 6,111,030 discloses amine-terminated polyamides for crosslinking epoxy compounds, polysulphides or cyanoacrylates. The disadvantage of these systems is the necessary use of a coreactant which may be toxic, such as isocyanates, acrylates or epoxide resins. Branched copolyamides comprising unsaturated ends have now been found. The crosslinking reaction is carried out by unsaturated groups which are easy to activate by irradiation (heat, UV, UV+ microwaves, β- or γ-rays). The unsaturated monoacids used as chain-limiting agents make possible the direct incorporation of the reactive principle in the polymer, which renders the product applicable without additional formulation.
The prior art has already disclosed polymers comprising unsaturated ends. Patent U.S. Pat. No. 6,680,264 discloses water-dispersible polyamides comprising unsaturated ends; these products are not HMAs. Patents EP 147 267 and WO 2003/087193 disclose linear polyamides comprising unsaturated ends. The advantage of having more than 2 unsaturated ends is that the crosslinking reaction results in the formation of a three-dimensional network, in comparison to a simple extension of the chains. The crosslinking is therefore much faster and more efficient.